I do not own P2atc but I am planning to download the 10 day trial. I have reviewed the documentation and some tutorial videos and have a few questions. I currently use x plane 11 and msfs2020 and plan to upgrade to x plane 12 during the next year. I have a Navigraph subscription, and use Simbrief and Sky Vector additionally to do flight planning. I assume P2atc will be compatible with x plane 12. Is there any extra charge for the upgrade  ? 

I think I read somewhere that P2atc works with any airport that has navigraph data. Is this true ?  

How compatible is P2atc with payware planes for x plane and mfs2020 ?

As I went through the documentation I noted that many of the functions like tuning radios can be done using the app or in the sim. I am a bit confused about flight planning. If I import a flight plan to P2atc will it appear in the gps or fmc in the sim or would I have to download to the sim and P2atc ?

P2A will work with all of those SIMs.

Yes, P2A uses AIRAC data and works with all airports.

Some aircraft don't allow external programs to do things like change the radio frequency or transponder.  Some others have a setting to enable/disable access from external programs.  If your aircraft is one of these, you can change frequencies and codes in the aircraft and P2A will almost always pick it up. 

When you import a plan into P2A, you File it with ATC and can export it for the SIM or aircraft you are flying.  Then you can load it using the process specified for that SIM and/or aircraft.  Using the export function and loading the result helps ensure that your aircraft has the same plan loaded as P2A is controlling from.

 P2A only loads flight plans into aircraft automatically for FSX and P3D after you get clearance.

Dave